Haire, 1973). The formation of the hydrated Pu0) is likely directly related to Put* concentration and inversely related to the acid concentration. Plutonium also tends to form many complexes with a range of stabilities. The strongest complexes are generally formed by reaction of organic ligands with Pu". However, many inorganic complexes and organic complexes of all valences may be stable under appropriate conditions. The presence of organic ligands in soils likely influences the equilibrium and concentration form of Pu in solution through complexation and subsequent inhibition of hydrolysis, polymerization, or disproportionation. It is these reactions in various highly complex combinations resulting from differences in source term, soil properties and processes that govern Pu solubility in soil and availability to plante. © FOUR OXIDATION STATES + Pu 3 pu*4 Pud,,”, Pud,”” (pure put!) ® DISPROPORTIONATION pu*4 + Pud, |= Pu 3 Pu,*@ ® Pu © Keeney and Wildung (1976), and limited information on the transuranic _\ + 41,0 = PulOH), + 4H + (Ke, = 10 -56 ) COMPLEX FORMATION 2Pu Fig. 3. On the basis of research with other trace metais, recently summarized by HYDROLYSIS +4 +4 Soil chemical reactions are important in governing the behavior of the various forms of Pu entering soil. Initially soluble forms entering soil have the potential for undergoing a range of chemical transformations (Fig. 1-3). Ineoluble Pu, such as high-fired oxide, entering soil likely will be solubilized with time, provided soluble, stable complexes are formed (Fig. 3). However, regardless of the form of Pu entering soil, its ultimate solubility will be controlled by its aqueous chemistry and by soil factors. Soil physiochemical properties may be expected to have complex, interdependent effects on Pu solubility. The long-term behavior of Pu in soil will be a function of the kinetics of these reactions. __\ +3 DIPAS= Pu, DTPA, log K = 26 Chemical reactions influencing plutonium behavior in soil 132 elements it may be concluded that the soil physicochemical parameters most important in influencing the solubility of the transuranics include: (1) solution composition, Fh and pH, (2) type and density of charge on soii colloids, and (3) reactive surface area. These phenomena will in turn be dependent upon soil properties, including particle size distribution, organic matter content, particle mineralogy, degree of aeration and microbial activity, The delineation of the influence of these factors on Pu solubility is difficult due to the complex chemistry of Pu. Perhaps the simplest approach to the study of the chemistry of Pu in soll is to direct initial attention to the factors influencing its solubility in soil. However, it is difficult to define Pu solubility in soil because solubility will depend upon the method of measurement and because solubility must be arbitrarily evaluated due to the sorption of Pu on submicron clay particles or to the formation of submicron particles of hydrated Pu oxide which are difficult to centrifuge and may pass membrane filters. These effects may be illustrated by comparison of the differences in the solubility of Pu in soils [100 days after amendment as Pu(NO3},,] as determined by water extraction and subsequent membrane filtration using membranes of different average pore sizes (Table 1). The major fraction of the Pu added was sorbed on the soil, as a maximum of 10% of the extracted Pu passed through the 5 u membrane. Successive filtration through membranes with decreasing pore size resulted in decreases in Pu concentration in the filtrate. Thus, Pu in the aqueous extract appeared to be in a wide range of particle sizes. Although membranes with pore sizes of 0.45 ») are commonly used to separate seluble 133